1. Field of the Invention
The present invention relates to an image forming system.
2. Description of Related Art
When an image forming apparatus cannot finish preparation of an image before a time when the image is written to a sheet, the image forming apparatus has to stop the sheet having been fed at a position in advance of a writing position (a position where writing processing is performed). As a result, a subsequent sheet catches up with the preceding sheet, which causes jam and reduces productivity.
In order to avoid such circumstance, a conventional image forming apparatus requires, as a condition of sheet feeding, that an output buffer (semiconductor memory) located immediately before an image forming unit stores an image corresponding to a sheet to be fed. In both-side printing operation, the condition of sheet feeding is that the output buffer stores images of both front and back sides (for example, Japanese Patent Application Laid-Open Publication No. 2001-310531).
Meanwhile, in recent years, an image forming system (series tandem system) configured by connecting two image forming apparatuses has been suggested. This image forming system is configured such that, in both-side printing operation, for example, the first image forming apparatus located upstream in sheet conveying direction prints a front surface of a sheet, and the second image forming apparatus located downstream in sheet conveying direction prints a back surface of the sheet. Since printing operations to the front and back sides of the sheet are shared by the respective image forming apparatuses, the productivity can be improved as compared with a case where both-side printing operation is performed with only one image forming apparatus. In general, such image forming system is applied to Production Printing (PP) machine, which pursues high productivity.
In the series tandem system, each of the first image forming apparatus and the second image forming apparatus is provided with an image processing unit for processing of preparation of an image for imaging and an image forming unit for forming the image on a sheet, and the image forming apparatuses have the image forming units at different positions with respect to the sheet conveying direction. Accordingly, preparation of an image to be formed in the second image forming apparatus provided downstream in the sheet conveying direction does not need to be completely finished, namely, does not need to be in a state ready to be used by the image forming section, in the stage of sheet feeding. In spite of the above fact, if the image forming apparatuses start feeding operation upon waiting for completion of preparation of the images for both of the image forming apparatuses, high degree of productivity cannot be achieved.